There are known compositions described, for example, in JP6-104792B as a thermosetting fluorine-containing powder coating composition. Also the present inventors have filed a patent application (Application No.: PCT/JP00/08802, Publication No. WO01/44390) with respect to a thermosetting fluorine-containing powder coating composition prepared from a fluorine-containing copolymer comprising tetrafluoroethylene (TFE), hexafluoropropylene (HFP), ethylene, hydroxybutyl vinyl ether (HBVE) and p-tertiary butyl vinyl benzoate (tBVBz). However there are no teachings with respect to amounts of ethylene and tBVBz in the copolymer and further thermal transition temperature and impact resistance of the obtained copolymer.
Also WO01/25346A1 and WO01/25354A1 disclose use of a TFE/HFP/ethylene/HBVE copolymer for a powder coating, and JP2000-26767A discloses use of a TFE/HFP/ethylene/perfluorobutenoic acid (PFBA) copolymer for a powder coating. Those patent publications disclose that impact resistance of a coating film at a coating thickness of 40 μm which is measured with an impact tester is 2.5 J.
With respect to a polymer for thermosetting powder coating, from the viewpoint of a mechanism of forming a coating film by heating and melting the polymer, it is necessary to increase a melt flowability of the polymer to increase smoothness of a coating film surface by decreasing a molecular weight of the polymer or decreasing a thermal transition temperature thereof. However if the molecular weight is decreased, there is a problem that impact resistance which is an essential characteristic of the coating film in its practical use is lowered. Also if the thermal transition temperature of the polymer is decreased, there is a problem that storage stability of an obtained powder coating composition is lowered or the powder is agglomerated in piping at coating.
The present inventors have made studies on those problems in a powder coating, and as a result, it was found that there is a close co-relation between an impact resistance of a coating film and a thermal transition temperature of a polymer. The relation is concretely such that when the thermal transition temperature of a polymer is increased, the impact resistance of a coating film is lowered. Accordingly a resin having a sufficiently high thermal transition temperature while having a practically satisfactory impact resistance (in evaluation at a coating thickness of 65 μm) has not been known.